The invention relates to a process for the preparation of an aromatic hydrocarbon mixture from paraffins having two, three or four carbon atoms per molecule, or from aliphatic hydrocarbon mixtures consisting more than 50%w of said paraffins, by using a catalyst containing a crystalline metal silicate of a special structure.
Olefins having two, three or four carbon atoms per molecule can be converted at a relatively low temperature and in high yields into aromatic hydrocarbon mixtures by contacting the olefins with a crystalline metal silicate of a special structure. The crystalline metal silicates concerned are characterized in that
(a) they have been prepared by crystallization from an aqueous mixture which, in addition to the components needed for synthesizing the silicate, comprises one or more compounds of a trivalent metal X chosen from the group formed by aluminum and iron, in such quantities that in the formula which represents the composition of the silicate expressed in moles of the oxides, the SiO.sub.2 /X.sub.2 O.sub.3 molar ratio of 10-500, and
(b) after one hour's calcination in air at 500.degree. C. they have an X-ray powder diffraction pattern in which the strongest lines are in four lines mentioned in Table A,
TABLE A ______________________________________ d(.ANG.) ______________________________________ 11.1 .+-. 0.2 10.0 .+-. 0.2 3.84 .+-. 0.07 3.72 .+-. 0.06 ______________________________________
A similar conversion into aromatic hydrocarbon mixtures of paraffins having two, three of four carbon atoms per molecule and of aliphatic hydrocarbon mixtures consisting more than 50%w of said paraffins (for the sake of brevity hereinafter referred to as "the present conversion") is much more difficult to achieve and requires considerably higher temperatures, which accounts for the important role played by cracking reactions and for the yields of C.sub.5.sup.+ hydrocarbons remaining low. During the present conversion hydrogen is released. In view of the growing demand for hydrogen for a variety of purposes, it is important that in the present conversion as much of the hydrogen as possible becomes available as molecular hydrogen instead of hydrogen-rich byproducts, such as methane. It has been found that the above-mentioned crystalline aluminum silicates show very low C.sub.5.sup.+ and H.sub.2 selectivities. The crystalline iron silicates have very low activity and, in addition, a low to very low C.sub.5.sup.+ selectivity. This is true both of the crystalline metal silicates with a high metal content (SiO.sub.2 /X.sub.2 O.sub.3 molar ratio &lt;100) and of the crystalline metal silicates with a low metal content (SiO.sub.2 /X.sub.2 O.sub.3 molar ratio &gt;100). A solution to the low activity and low selectivity problem of the present conversion may be found in a two-step process in which in the first step the paraffins are converted into olefins by dehydrogenation, followed in the second step by conversion of these olefins over the above-mentioned crystalline aluminum and iron silicate catalysts. Naturally, for carrying out the present conversion on a technical scale a one-step process is much to be preferred to a two-step process; therefore, in spite of the above-described disappointing results with the crystalline aluminum and iron silicates of a special structure (as characterized in Table A), an extensive investigation was carried out to determine whether by introducing certain changes in the composition of the metal silicates without affecting their special structure products can be prepared which are suitable for use as catalysts for carrying out the present process in a single step. The investigation has surprisingly shown that compositions containing both gallium and a crystalline metal silicate of the afore-mentioned special structure--provided that they meet certain conditions with regard to the gallium content and the way in which the gallium has been incorporated into the composition, as well as to possible subjection of the gallium-containing composition to one or more two-step treatments--are excellently suitable for use as catalysts for carrying out the present conversion in a single step. Either per se or after having been subjected to one or more two-step treatments said gallium-containing catalysts have both high activity and high C.sub.5.sup.+ and H.sub.2 selectivities.
The gallium-containing catalysts which, --whether after one or more two-step treatments or not--are eligible for use in the present conversion can be arranged in the following two classes based on the manner in which the gallium is present in the catalysts:
I. Catalysts containing a crystalline gallium silicate which is characterized in that
(a) it has been prepared by crystallization from an aqueous mixtures which, in addition to the components needed for synthesizing the silicate, comprises one or more gallium compounds and, if desired, one or more compounds of a trivalent metal Y chosen from the group formed by aluminum, iron, cobalt and chromium, in such quantities that in the formula which represents the composition of the silicate expressed in moles of the oxides, the SiO.sub.2 /Ga.sub.2 O.sub.3 molar ratio is 25-250 and the Y.sub.2 O.sub.3 /Ga.sub.2 O.sub.3 molar ratio is lower than 1, and
(b) after one hour's calcination in air at 500.degree. C. it has an X-ray powder diffraction pattern in which the strongest lines are the four lines mentioned in Table A.
II. Catalysts containing gallium supported on a carrier and a crystalline metal silicate which is characterized in that
(a) it has been prepared by crystallization from an aqueous mixture which, in addition to the components needed for synthesizing the silicate, comprises one or more compounds of a trivalent metal Y chosen from the group formed by aluminum, iron, cobalt and chromium and, if desired, one or more gallium compounds, in such quantities that in the formula which represents the composition of the silicate expressed in moles of the oxides, the SiO.sub.2 /(Y.sub.2 O.sub.3 +Ga.sub.2 O.sub.3) molar ratio is 10-500 and the Ga.sub.2 O.sub.3 /Y.sub.2 O.sub.3 molar ratio is lower than 1, and
(b) after one hour's calcination in air at 500.degree. C. it has an X-ray powder diffraction pattern in which the strongest lines are the four lines mentioned in Table A,
in which catalysts the quantity of gallium which occurs supported on a carrier firstly amounts to 0.3-10%w, calculated on the sum of the quantity of crystalline metal silicate present in the catalyst and the quantity of other material used as carrier for the gallium which may be present in the catalyst, and secondly amounts to 1-10%w, calculated on the amount of crystalline metal silicate present in the catalyst.
As regards the composition, the main difference between the catalysts belonging to Classes I and II is the fact that the gallium present in the catalysts belonging to Class I occurs in the crystalline silicate of a special structure alone and has been incorporated therein during the preparation of the silicate by crystallization from an aqueous mixture containing one or more gallium compounds, whereas in the case of the catalysts belonging to Class II at least part of the gallium present therein occurs as a deposit on a carrier. A carrier for the gallium that may suitably be used is the crystalline silicate of a special structure present in the catalysts belonging to Class II, onto which the gallium has been deposited for instance by impregnation or ion exchange. In the catalysts belonging to Class II the gallium occurring may be partly or wholly deposited on a conventional carrier, such as silica, the gallium-loaded carrier being present in the catalyst in admixture with a crystalline silicate of a special structure. Just as in the case of the crystalline silicates present in the catalysts belonging to Class I it holds that the aqueous mixture from which they are prepared by crystallization may contain, in addition to one or more gallium compounds, a minor quantity of one or more compounds of trivalent metals Y, so in the case of the crystalline silicates present in the catalysts belonging to Class II it holds that the aqueous mixture from which they are prepared by crystallization may contain, in addition to one or more compounds of a trivalent metal Y, a minor quantity of one or more gallium compounds.
With regard to the differences in performance existing between the catalysts belonging to class I when used in the present conversion, these catalysts may be further divided into a class IA and a class IB, depending on the SiO.sub.2 /Ga.sub.2 O.sub.3 molar ratio of the crystalline gallium silicate present therein. The catalysts belonging to class IA are characterized in that the crystalline gallium silicate that they contain has a SiO.sub.2 /Ga.sub.2 O.sub.3 molar ratio lower than 100. In the catalysts belonging to class IB the SiO.sub.2 /Ga.sub.2 O.sub.3 molar ratio of the crystalline gallium silicate present therein is at least 100.
It has been found that the catalysts belonging to class IA have a very high activity as well as very high H.sub.2 and C.sub.5.sup.+ selectivities. Consequently, they are excellently suitable to be used per se as catalysts for carrying out the present conversion.
It has been found that the catalysts belonging to class IB have a relatively low activity and a low to very low C.sub.5.sup.+ selectivity. This makes them less suitable for use in carrying out the present conversion such as they are. However, further investigation into this subject has revealed that the performance of these catalysts in carrying out the present conversion can be greatly enhanced by subjecting them once or several times to a two-step treatment. This treatment leads to a substantial increase in activity and H.sub.2 and C.sub.5.sup.+ selectivities and renders it possible, starting from the catalysts belonging to class IB which as such are not suitable for use in carrying out the present conversion, to produce catalysts whose activity and H.sub.2 and C.sub.5.sup.+ selectivities lie at a level comparable to that of the catalysts belonging to class IA. The investigation has further shown that if the treatment found for the catalysts belonging to class IB is applied to the catalysts belonging to class IA (which in themselves have a very high activity and very high H.sub.2 and C.sub.5.sup.+ selectivities), a considerable additional gain in activity and C.sub.5.sup.+ selectivity can be achieved for the latter catalysts as well. The invention therefore in the first place relates to carrying out the present conversion by using a catalyst belonging to class I (class IA as well as class IB) which has been subjected once or several times to a two-step treatment.
It has been found that the catalysts belonging to class II have a very low C.sub.5.sup.+ selectivity. This renders them rather unsuitable to be used per se in carrying out the present conversion. However, continued investigation into this subject has shown that the performance of these catalysts when carrying out the present conversion can be much improved by subjecting them once or several times to a two-step treatment. This treatment results in a substantial increase of the C.sub.5.sup.+ selectivity. In addition the treatment leads to enhancement of the activity and H.sub.2 selectivity of the catalysts. Using a two-step treatment renders it possible, starting from catalysts belonging to class II, which on account of their low C.sub.5.sup.+ selectivity are unsuitable to be used per se in carrying out the present conversion, to obtain catalysts which are very suitable for the purpose. The invention therefore further relates to carrying out the present conversion by using a catalyst belonging to class II which has been subjected once or several times to a two-step treatment.
The liquid hydrocarbon mixtures obtained in the present conversion boil substantially in the gasoline range and have a very high octane number. They are therefore excellently suitable for use as motor gasoline or as mixing components for motor gasolines.
The treatment of a catalyst belonging to class I involves subjecting the catalyst once or several times to a two-step treatment comprising a step in which the catalyst is contacted for at least 15 minutes and at a temperature of 400.degree.-650.degree. C. with a reducing gas which contains at least 20%v hydrogen, followed by a second step in which the catalyst is contacted for at least 15 minutes and at a temperature of 350.degree.-700.degree. C. with an oxidizing gas containing at least 5%v oxygen. According as the catalysts belonging to class I are subjected to the two-step treatment more often, their performance in the present conversion will improve. This improvement progresses until a certain maximum level has been reached, where further repetition of the two-step treatment ceases to produce any effect. The minimum number of times that catalysts belonging to class I should be subjected to the two-step treatment in order to raise their performance in the present conversion to an acceptable level is dependent on the SiO.sub.2 /Ga.sub.2 O.sub.3 molar ratio of the silicate present therein, and for catalysts in which the silicate has a SiO.sub.2 /Ga.sub.2 O.sub.3 molar ratio higher than 110 it is given by the formula ##EQU1## wherein n represents the minimum number of two-step treatments and m the SiO.sub.2 /Ga.sub.2 O.sub.3 molar ratio of the silicate. As regards the catalysts belonging to class I, it has surprisingly been found that the number of times that the two-step treatment has to be carried out in order to carry their performance to a certain desired high level can be considerably decreased if, before being subjected to a succession of two-step treatments, the catalysts are exposed to calcination at a temperature of 600.degree.-1000.degree. C. have to be subjected to the two-step treatment in order to carry their performance in the present conversion to an acceptable level again depends on the SiO.sub.2 /Ga.sub.2 O.sub.3 molar ratio of the silicate present therein, and for catalysts in which the silicate has a SiO.sub.2 /Ga.sub.2 O.sub.3 molar ratio higher than 130 it is given by the formula ##EQU2## wherein n and m have the meanings mentioned hereinbefore. In this connection it should be noted that when the above formulae produce a value for n which can be expressed as the sum of a natural number N and a fractional number smaller than 1, the minimum number of times that the catalyst should be subjected to the two-step treatment is N+1.
Carrying out the present conversion by using a catalyst belonging to class I which has been subjected once or several times to the two-step treatment described hereinbefore forms the subject matter of the present patent application.
Carrying out the present conversion by using a catalyst belonging to class II which has been subjected once or several times to a two-step treatment forms the subject matter of Netherlands patent application No. 8302788.